1. Field of the Invention
The present invention relates to a data receiving system in which adverse influence of an interference signal is removed in a data transmission to improve a frequency utilizing efficiency.
2. Description of the Related Art
In cases where an intensity level of a received signal is low in a mobile communication, a ratio of a noise level to a receiving level of a desired signal often becomes high. In this case, a noise is included in an audio signal generated in an analog circuit, and a bit error occurs in a digital communication. Also, in cases where an undesired signal having a high level exists as an interference signal in the same frequency band as that of a desired signal or in a frequency band adjacent to that of the desired signal, even though a receiving level of the desired signal is high, a receiving quality for the desired signal deteriorates in the same manner as that in cases where a receiving level of the desired signal is low. The undesired signal functioning as an interference signal is mainly transmitted from an undesired base station, and it has been thought until recent years that the removal of the undesired signal having a high level is impossible. Therefore, the arrangement of base stations and the allocation of frequencies cannot be arbitrarily performed, and a circuit design becomes complicate. In other words, the existence of an undesired signal having a high intensity is a large obstacle to improve the frequency utilizing efficiency. In recent years, a method for removing the interference signal has been reported. Two representative conventional removing methods are described hereinafter.
2.1. PREVIOUSLY PROPOSED ART:
FIG. 1 is a block diagram of a signal generating system. The signal generating system is used as a model of propagation path.
As shown in FIG. 1, a desired signal X(t) generated in a desired signal generating unit 1 is generally deformed by the influence of the fading and delay occurring in a propagation path. To model the influence of the fading and delay on the desired signal X(t), the desired signal X(t) is multiplied by a complex number A0 in a weighting unit 7 to generate a direct component (or a non-delayed component) A0*X(t) based on the influence of the fading, and the desired signal X(t) is delayed by a delayed time .tau. in a delaying unit 3 to generate a delayed signal X(t-.tau.) based on the influence of the delay. Because the delayed signal receives the influence of the fading regardless of the direct component, the delayed signal is multiplied by a complex number A1 in a weighting unit 8 to generate a delayed component A1*X(t-.tau.) based on the influence of the fading and delay. Also, an interference signal Y(t) generated in an interference signal generating unit 2 is generally deformed by the influence of the fading and delay occurring in another propagation path in the same manner as the desired signal X(t). Therefore, the interference signal Y(t) is multiplied by a complex number B0 in a weighting unit 9 to generate a direct component B0*Y(t) based on the influence of the fading, and the interference signal Y(t) is delayed by the delayed time .tau. in a delaying unit 4 and is multiplied by a complex number B1 in a weighting unit 10 to generate a delayed component B1*Y(t-.tau.) based on the influence of the fading and delay. Thereafter, the direct component A0*X(t), the delayed component A1*X(t-.tau.), the direct component B0*Y(t) and the delayed component B1*Y(t-.tau.) are summed in an adder 15 to generate a first receiving signal Sa.
Also, in the same manner, the desired signal X(t) is multiplied by a complex number C0 in a weighting unit 11 to generate a direct component C0*X(t) based on the influence of the fading, and the desired signal X(t) is delayed by the delayed time .tau. in a delaying unit 5 and is multiplied by a complex number C1 in a weighting unit 12 to generate a delayed component C1*X(t-.tau.) based on the influence of the fading and delay. Also, the interference signal Y(t) is multiplied by a complex number DO in a weighting unit 13 to generate a direct component D0*Y(t) based on the influence of the fading, and the interference signal Y(t) is delayed by the delayed time .tau. in a delaying unit 6 and is multiplied by a complex number D1 in a weighting unit 14 to generate a delayed component D1*Y(t-.tau.) based on the influence of the fading and delay. Thereafter, the direct component C0*X(t), the delayed component C1*X(t-.tau.), the direct component D0*Y(t) and the delayed component D1*Y(t-.tau.) are summed in an adder 16 to generate a second receiving signal Sb. Therefore, the receiving signals Sa and Sb are generated in a signal generating system 18.
The receiving signals Sa and Sb are expressed according to equations (1) and (2). EQU RA(t)=A0*X(t)+A1*X(t-.tau.)+B0*Y(t)+B1*Y(t-.tau.) (1) EQU RB(t)=C0*X(t)+C1*X(t-.tau.)+D0*Y(t)+D1*Y(t-.tau.) (2)
Here, the symbol RA(t) denotes the first receiving signal Sa, and the symbol RB(t) denotes the second receiving signal Sb. Though the complex numbers A0, A1, B0, B1, C0, C1, D0 and D1 are respectively expressed by a fixed value, the complex numbers are actually time-changed.
FIG. 2 is a block diagram of a first conventional data receiving system for receiving data signals transmitted from the signal generating system shown in FIG. 1.
As shown in FIG. 2, a plurality of first receiving signals Sa are received one after another by an antenna 21, and each signal Sa is multiplied by a first coefficient in a forward tap 23. Also, a plurality of second receiving signals Sb are received one after another by another antenna 22, and each signal Sa is multiplied by a second coefficient in another forward tap 24. Thereafter, the first multiplied signal Sa and the second multiplied signal Sb are summed in an adder 26. In this case, the first and second coefficients independent of each other are adjusted to maximize a level ratio of a desired component composed of the components A0*X(t), A1*X(t-.tau.), C0*X(t) and C1*X(t-.tau.) to an interference component composed of the components B0*Y(t), B1*Y(t-.tau.), D0*Y(t) and D1*Y(t-.tau.). Therefore, the influence of the interference signal Y(t) is suppressed. Also, to remove the delay components A1*X(t-.tau.) and C1*X(t-.tau.) of the desired signal X(t), a delay component generated in a weighting unit 25 is subtracted in the adder 26 from the sum of the first and second multiplied signals Sa and Sb. That is, an output of the adder 26 obtained in a current period is quantized to a series of binary values in an identifying unit 28 to generate a demodulated signal Sx, the demodulated signal Sx is delayed by one symbol time T in a delaying unit 29, the demodulated signal Sx delayed is weighted by a gain in the weighting unit 25, and the demodulated signal Sx delayed and weighted is subtracted from a sum of other first and second multiplied signals Sa and Sb in a succeeding period. Also, the series of binary values of the identifying unit 28 is subtracted from the output of the adder 26 in a subtracting unit 27 to calculate an error Ex. In a first conventional data receiving system 30, the gain of the weighting unit 25, the coefficients of the forward taps 23 and 24 are adjusted to minimize a squared value of the error Ex each time a pair of first and second receiving signals Sa and Sb are received.
Accordingly, the first conventional data receiving system 30 functions as an equalizing unit in which the components B0*Y(t), B1*Y(t-.tau.), D0*Y(t) and D1*Y(t-.tau.) relating to the interference signal Y(t) are suppressed and the delay components A1*X(t-.tau.) and C1*X(t-.tau.) of the desired signal X(t) are removed, and the demodulated signal Sx relating to the direct components A0*X(t) and C0*X(t) of the desired signal X(t) can be obtained.
FIG. 3 is a block diagram of a second conventional data receiving system for receiving data signals transmitted from the signal generating system shown in FIG. 1.
As shown in FIG. 3, because a diversity reception is performed in a second conventional data receiving system 40, a plurality of first receiving signals Sa are received one after another by an antenna 41, and a plurality of second receiving signals Sb are received one after another by another antenna 42. In a maximum likelihood sequence estimating (MLSE) equalizing unit 60, in cases where a quadri-phase shifting keying (QPSK) method is adopted as a modulation method, four desired conditions are considered for the desired signal X(t) to generate all signals which each are likely received by the antennas 41 and 42 as the desired signal X(t), and four interference conditions are considered for the interference signal Y(t) to generate all signals which each are likely received by the antennas 41 and 42 as the interference signal Y(t). The desired conditions are transmitted one after another in arbitrary order to a first circuit assuming unit 51, a first desired signal reproducing unit 53, a second circuit assuming unit 52 and a second desired signal reproducing unit 54, and the interference conditions are transmitted one after another in arbitrary order to the first circuit assuming unit 51, a first interference signal reproducing unit 55, the second circuit assuming unit 52 and a second interference signal reproducing unit 56. Therefore, there are 16 (4*4) types of combinations of the desired and interference conditions.
In the reproducing unit 53, four types of likely desired signals Xi(t) and Xi(t-.tau.) (i=1 to 4) are produced one after another according to the desired conditions. Also, four types of complex numbers A0i and A1i which each are likely used in the weighting units 7 and 8 of the signal generating system 18 are assumed in a first circuit assuming unit 51 according to the desired conditions. Thereafter, the likely desired signals Xi(t) and Xi(t-.tau.) are multiplied by the complex numbers A0i and A1i in a first multiplying unit 47 to generate four types of likely desired signals A0i*Xi(t)+A1i*Xi(t-.tau.) as replicas of the desired signal A0*X(t)+A1*X(t-.tau.) considering the influence of the fading and delay. In the same manner, four types of likely interference signals Yj(t) and Yj(t-.tau.) (j=1 to 4) are produced one after another in the reproducing unit 55 according to the interference conditions, and four types of complex numbers B0j and B1j which each are likely used in the weighting units 9 and 10 of the signal generating system 18 are assumed in the first circuit assuming unit 51 according to the interference conditions. Thereafter, the likely interference signals Yj(t) and Yj(t-.tau.) are multiplied by the complex numbers B0j and B1j in a second multiplying unit 49 to generate four types of likely interference signals B0j*Yj (t)+B1j*Yj(t-.tau.) as replicas of the interference signal B0*Y (t)+B1*Y(t-.tau.) considering the influence of the fading and delay. Thereafter, each of the likely desired signals A0i*Xi (t)+A1i*Xi(t-.tau.) and each of the likely interference signals B0j*Yj(t)+B1j*Yj(t-.tau.) are summed in an adder 45 to generate sixteen types of first likely receiving signals RAk(t) (k=1 to 16) one after another, each of the first likely receiving signals RAk(t) is subtracted from a first receiving signal Sa currently received by the antenna 41 in a subtracting unit 43 to obtain sixteen first errors one after another, and each of the first errors is squared in a squaring unit 57 to obtain sixteen first squared values.
In the same manner, four types of likely desired signals Xi(t) and Xi(t-.tau.) are produced in the reproducing unit 54, four types of complex numbers C0i and C1i which each are likely used in the weighting units 11 and 12 of the signal generating system 18 are assumed in a second circuit assuming unit 52 according to the desired conditions, and the likely desired signals Xi(t) and Xi(t-.tau.) are multiplied by the complex numbers C0i and C1i in a third multiplying unit 48 to generate four types of likely desired signals C0i*Xi(t)+C1i*Xi(t-.tau.) as replicas of the desired signal C0*X(t)+C1*X(t-.tau.) considering the influence of the fading and delay. Also, four types of likely interference signals Yj(t) and Yj(t-.tau.) are produced in the reproducing unit 56, four types of complex numbers D0j and D1j which each are likely used in the weighting units 13 and 14 of the signal generating system 18 are assumed in the second circuit assuming unit 52 according to the interference conditions, and the likely interference signals Yj(t) and Yj(t-.tau.) are multiplied by the complex numbers D0j and D1j in a fourth multiplying unit 50 to generate four types of likely interference signals D0j*Yj (t)+D1j*Yj(t-.tau.) as replicas of the interference signal D0*Y (t)+D1*Y(t-.tau.) considering the influence of the fading and delay. Thereafter, each of the likely desired signals C0i*Xi (t)+C1i*Xi(t-.tau.) and each of the likely interference signals D0j*Yj(t)+D1j*Yj(t-.tau.) are summed in an adder 46 to generate sixteen types of second likely receiving signals RBk(t), each of the second likely receiving signals RBk(t) is subtracted from a second receiving signal Sa currently received by the antenna 42 in a subtracting unit 44 to obtain sixteen second errors, and each of the second errors is squared in a squaring unit 58 to obtain sixteen second squared values.
Thereafter, because the diversity reception is performed, each of the first squared values and each of the second squared values are added in an adder 59 to generate sixteen summed values one after another, and a minimum summed value is selected from the summed values. Therefore, it is ascertained that a likely desired signal Xi(t) and a likely interference signal Yj(t) corresponding to the minimum summed value are appropriate as the desired signal X(t) and the interference signal Y(t). Thereafter, the signals Xi(t) and Yj(t) corresponding to the minimum summed value are output from a second conventional data receiving system 40 as the desired signal X(t) and the interference signal Y(t). Therefore, the desired signal X(t) can be obtained from the first and second receiving signals Sa and Sb.
Accordingly, the adverse influence of the fading and delay modeled in the signal generating system 18 can be reduced, and the deterioration of the desired signal X(t) can be improved.
2.2. PROBLEMS TO BE SOLVED BY THE INVENTION:
However, there are many drawbacks in the first and second conventional data receiving systems 30 and 40 as follows.
In the second conventional data receiving system 40, though the deterioration of the desired signal X(t) can be improved, there is drawbacks that a large volume of calculation is required and it is difficult to adopt the second conventional data receiving system 40.
In the first conventional data receiving system 30, though a volume of calculation is small, the improvement of the desired signal X(t) degraded by the existence of the interference signal Y(t) is inferior to that in the second conventional data receiving system 40. In particular, in cases where the interference wave Y(t) and the delayed interference wave Y(t-.tau.) exist with the desired signal X(t), the improvement of the desired signal X(t) is considerably inferior to that in the second conventional data receiving system 40.